Electromagnetic forming is a method of forming sheet metal or thin walled tubes that is based on placing a work-coil in close proximity to the metal to be formed and running a brief, high intensity current pulse through the coil. If the metal to be formed is sufficiently conductive the change in magnetic field produced by the coil will produce eddy currents in the work piece. These currents also have associated with them a magnetic field that is repulsive to that of the coil. This natural electromagnetic repulsion is capable of producing very large pressures that can accelerate the work piece at high velocities (typically 10-200 meters/second). This acceleration is produced without making physical contact to the work piece. The electrical current pulse is usually generated by the discharge of a capacitor bank. This field has been developed by many individuals and companies and is widely used for the forming and assembly of tubular and sheet work pieces. Several excellent reviews of the field are available, including Moon, F. C., Magneto-Solid Mechanics, ASTME, High Velocity Forming of Metals, revised edition (1968); Plum, M. M., Electromagnetic Forming, Metals Handbook, Maxwell Laboratories, Inc., pp. 644-653; and Belyy, I. V., Fertik, S. M. and Khimenko, L. T., Electromagnetic Metal Forming Handbook, Khar'kov State University, Khar'kov, USSR (1977) (Translation from Russian by M. M. Altoynova 1996), all of which are hereby incorporated herein by reference. Examples of prior art patents involving electromagnetic forming include U.S. Pat. No. 4,947,667 to Gunkel et al., U.S. Pat. No. 4,531,393 to Weir aet al., U.S. Pat. No. 5,353,617 to Cherian et al., U.S. Pat. No. 3,998,081 to Hansen et al., U.S. Pat. No. 5,331,832 to Cherian et al., U.S. Pat. No. 5,457,977 to Wilson, U.S. Pat. No. 4,619,127 to Sano et al., U.S. Pat. No. 4,473,862 to Hill, U.S. Pat. No. 4,151,640 to McDermott et al. and U.S. Pat. No. 5,016,457 to Richardson et al., all of which are hereby incorporated herein by reference.
Electromagnetic forming can be carried out on a wide range of materials and geometries within some fundamental constraints. First, the material must be sufficiently electrically conductive to exclude the electromagnetic field of the workcoil. The physics of this interaction have been well characterized.
Another key constraint is coil strength. Generally equal and opposing external forces will exist on the work-coil and work piece. In addition, the current that runs through the coil can induce large internal transient forces that are separate with the repulsion from the work piece. These can also lead to coil failure. In the development of electromagnetic forming coils the issue of robustness has generally been addressed by wrapping what is often called a `field-shaper` with many windings of coil from the power source. The main mechanical forces are transmitted through the massive field shaper which accepts the pressure from the work piece acceleration. This concept is used to make the `wafer coils` which are generally acknowledged to produce the highest pressures from production coils designed for thousands to hundreds of thousands of operations without a rebuilding procedure. In many other designs the deformation of the work coil or fracture of insulation and arcing usually causes the coils to fail after a number of operations. That number is set roughly by the construction of the coil and the average energy of the operations performed.
To date, all of the coils that have been reported to have been used for electromagnetic forming typically have had roughly cylindrical symmetry, such as shown in schematic form in FIG. 1. FIG. 1 shows a standard unidirectional coil through which the current pulse is passed from the positive pole 11 to the negative pole 12. The coil 10 is supplied with a current pulse from a power source which typically is a simple bank of capacitors with appropriate charging circuitry and discharge bus work, as is well-known in the art (not shown).
The most common such coils are those based on simple solinoidal windings used for the expansion or compression of tubes. Less common, but still abundantly mentioned in the literature, are `pancake coils` that use a flat spiral geometry. Such coils are commonly used to form bulge-like features in flat metal sheets. These coils do suffer from limited strength and as a result of this a number of specialized coils using massive field shapers have been developed and are taught in the art. Both field shaper geometry and the standard flat spiral geometry are limited in that the magnetic pressure drops to zero at the center of the actuator.
As shown in FIG. 1A, a cross-section along line 1A--1A of FIG. 1, the magnetic field produced in the work-force area has a area of weakness in the center of the work-force area. Careful analyses have been performed on the flat spiral coil (G. Fenton, N. Takatsu, M. Kato, K. Sato and T. Tobe, JSME International Journal, 31, 142 (1988)) which show that the magnetic pressure is actually a maximum at the outer edge of the coil. This is a distinct disadvantage in most practical cases as the maximum displacement and force are generally desired at the center of work-force area adjacent the coil.
Secondly, such a geometry is disadvantageous because all of these coils are typically based on circular symmetry. This makes it very difficult to envision ways to form many shapes such as those including elongated features or those with complicated shapes which may be extended from the plane of the work piece.
Accordingly, it is desirable to be able to produce electromagnetic actuators that can provide maximum force and displacement and force at the center of the actuator coil, and which can be produced in robust arrangements that resist and maximize mechanical stress in the forming operation. It is also desirable to provide an actuator that produces a relatively uninterrupted magnetic field over the region where forming is desired.
It is also advantageous to be able to produce electromagnetic actuators that can be conveniently applied to the formation of elongated metal pieces, as well as the formation of other metal shapes of a wide variety of geometries.
It is also an object of the present invention to eliminate some of the drawbacks in the prior art by allowing one to tailor the spatial distribution of pressure more effectively and by permitting the building of stronger, more robust coils (i.e., typically of thicker conduit cross-section) to balance internal magnetic forces.
The present invention also has as its goal to allow one to produce electromagnetic actuators that provide more uniform force distribution and/or force distribution that are tailored to suit the geometry of the component being formed.
The present invention also allows such actuators to be more easily incorporated into, and used with, molds, forming dies and tool bodies.
In view of the following disclosure, other advantages of the invention, and the solution to other problems using the invention, may become apparent to one of ordinary skill in the art.